Flow rate measurement derived from differential pressure readings is common and is found in many types of fluid flow meters. Pitot tubes, for instance, sense the upstream total pressure of a flowing fluid and the downstream static pressure to produce a differential pressure value that is used to develop the rate of flow of the fluid that is impacting the pitot tube. Such devices include pressure ports leading to fluid plenums in the pitot tube body. Impulse lines then transmit the fluid pressure to a transducer that translates the respective fluid pressures to electrical signals that are transmitted to flow calculating devices.
One of the biggest difficulties with flow meters having plenums and impulse lines is their propensity for leakage and clogging. This problem requires frequent monitoring to insure that the lines are bled, the interconnecting manifold is zeroed and the associated transmitter is properly calibrated.
As subsequently described, the solution to the leakage problem proposed by the present invention is to substitute electrical connections for the plenums and impulse lines that conduct fluid. Electrical strain gages are the logical choice to convert fluid pressures and the resulting meter strain to electrical signals. Strain gages have been used in target flow meters for some time. Target flow meters provide flow measurement by sensing the fluid force acting on a target suspended in a flow stream. A typical strain gage target flow meter comprises a sensing element that includes a target rod, a calibrated target disk, mounting base, protective case and a sensing tube to which are attached electrical strain gages. In some applications four strain gages are attached to the sensing tube, two on the leading side of flow and two on the trailing side of the flow. Fluid flow produces a strain on the sensing tube, compressing the leading side strain gages and tensing the trailing side strain gages, causing their resistance to decrease and increase respectively. Target meters are limited in their use because the disc will only interact with a small portion of the flow stream. Therefore accuracy and range suffer and the signal is weak. The relatively small area of interaction with the fluid flow also eliminates the possibility of obtaining the averaging effect that is a prominent feature of a diametric pitot tube.
Another type of strain gage flow meter is shown in U.S. Pat. No. 4,604,906. The flow meter of that disclosure uses an airfoil shaped sensing element which is aligned with the flow of fluid. The flow responsive thrust on the sensing element created by differential pressure on the two sides of the airfoil is transverse to the direction of fluid flow.
Strain gages have also been employed with vortex shedding flow meters, such as the one disclosed in U.S. Pat. No. 4,791,818. In that disclosure frequency of the shedding vortices is measured by a cantilevered beam equipped with one or more strain gages.
Therefore, the principal object of the present invention is to provide a differential pressure flow meter that does not require fluid receiving plenums or impulse lines to convey pressure data.
A second objective of the invention is to apply the advantages of strain gage technology to the well known and widely accepted features of several types of fluid flow meters such as, for example, an averaging pitot tube, an orifice plate and a V-cone while at the same time eliminating the problems inherent with the use of impulse lines.
Other and further objects, features and advantages of the present invention will become apparent upon a reading of the following specification taken in conjunction with the accompanying drawings.